This invention relates to a diaphragm for an electroacoustic transducer which is adapted for those such as speakers, microphones and the like.
Recently, it is proposed to use carbonfiber as a material for composing a diaphragm for an electroacoustic transducer which is, for example, adapted for speakers. The carbonfiber is used in order to increase rigidity of the diaphragm. Diaphragms, which are actually used prior to this invention, may be exemplified as follows.
(1) The diaphragms produced by the steps of mixing carbonfibers and pulpfibers to form a shape of a diaphragm, impregnating the formed diaphragm with thermosetting resins, and thereafter pressing same under heat.
(2) The diaphragms produced by the steps of mixing thermoplastic resins such as polyplopylene with carbonfibers to form a sheet, and then forming the sheet into a shape of a diaphragm by vacuum forming or molding the mixed material to be the diaphargm by injection.
(3) The diaphragms produced by the steps of impregnating a cloth of carbonfiber with thermosetting resins to be prepreg and pressing same into a shape of a diaphragm.
The above-mentioned diaphragms in the prior art, however, have the following drawbacks.
In the case of Item (1) above, the chracteristics of high elasticity as carbonfiber is not utilized sufficiently and the mixed or composite material including 50 weight percents of the carbonfibers indicates Young's modulus of at most 5.times.10.sup.10 dyn/cm.sup.2. Because, Young's modulus has a peak value with respect to the mixing ratio of the carbonfibers and pulpfibers and therefore even if the mixing amount of the carbonfibers is increased, it does not contribute to increasing of Young's modulus but rather brings about decreasing of Young's modulus.
In the case of Item (2) above, the mixing amount of the carbonfibers is limitted due to the size of an injection nozzle and fluidity of the mixture when it is formed into a sheet. For example, in the case of a sheet having thickness of 0.3-0.5 mm, the mixing ratio of the carbonfibers becomes 20 weight percents at the most.
Moreover, the mixing amount of the carbonfibers is further limitted in consideration of manufacturing processes of vaccum forming.
On the other hand, in the case of injection molding, the mixing amount of the carbonfibers becomes 15 weight percents in consideration of fluidity of the mixture. Therefore, it cannot increase rigidity of the diaphragm in the case of Item (2) above.
In the case of Item (3) above, rigidity of the diaphragm may be increased greatly in comparison with Item (2), but it is difficult to form a conical or dome shape from a plane cloth which is woven. Because, carbonfiber originally provides elasticity a little and only texture slippages have to be utilized when a plane cloth is formed into a solid shape such as conical and dome shapes. However, such texture slippages are very small once it has been woven as a cloth. Thus, it is necessary to utilize the texture slippages when forming the diaphragm of conical or dome shape and to this end, a number of processes including preforming are necessary. As the result, manufacturing cost becomes very expensive and further it is difficult to form a deep cone diaphragm or a cone diaphragm of complex shape having ribs at the intermediate portion thereof to form corrugation.